The Title

Most would say the title "Of Mice and Men" simply refers to what the book is about; everyone, whether they be royal or poor. They'd also say how it is a very, very loose connection to Lennie's infatuation with soft things.
However, there is strong evidence pointing towards a Scottish poet from the 1700's by the name of Robert Burns. He wrote a poem called To A Mouse, in which the second-to-last line states, "The best laid plans of mice and men gang aft agley." In modern-day English, this says that no matter how carefully you prepare, things sometimes go wrong. This is the main theme of the book and even bears the title of the book.
With this evidence, it would not surprise me in the slightest if John Steinbeck was familiar with the works of Robert Burns.